When meiosis begins, replicated sister chromatids are held together by sister chromatid cohesion (SCC) mediated by the cohesin complex. Subsequently, homologous chromosomes become linked through crossover recombination. Genome copy number (ploidy) is reduced when stepwise proteolysis of a single cohesin subunit, the ?-kleisin, triggers two rounds of chromosome segregation: homologs separate first, then sisters. Pioneering work in budding yeast showed that the successive separation of homologs and sisters requires that the mitotic kleisin Scc1 be replaced by the meiosis-specific kleisin Rec8; only Rec8 can be cleaved in two steps. However, Rec8 is not the sole meiotic kleisin in all organisms. I showed that C. elegans meiosis requires two nearly identical and completely functionally redundant ?-kleisins, called COH-3 and COH-4 (henceforth, COH-3/4), in addition to REC-8. Remarkably, REC-8 and COH-3/4 cohesins differ in nearly every property, including how they associate with meiotic chromosomes, how they become cohesive once bound to chromosomes, and when, where, and how they dissociate from chromosomes in prophase of meiosis I, prior to the proteolytic cleavage of the kleisin described above. It has recently been shown that plants and mammals utilize sets of meiotic kleisins that appear functionally similar to those we identified in nematodes, and that the plant and mammalian kleisins endow meiotic cohesins with divergent properties as occurs in nematodes. Because defects in meiotic SCC are thought to be a major factor contributing to human miscarriage, birth defects, and infertility, we propose to identify the molecular mechanisms that underlie the functional differences between meiotic cohesins that use different kleisins. REC-8 and COH-3/4 are expressed at different times, and we will determine whether expression timing underlies any functional differences. Our preliminary evidence suggests that numerous kinases involved in cell cycle progression, including ATM, ATL, CHK-2, Polo, and Aurora B, differentially regulate REC-8 and COH-3/4 cohesin, and we propose to identify kleisin-specific post- translational modifications and determine their functional importance in SCC establishment and proteolysis- independent cohesin removal. We will also determine whether pathways other than the canonical WAPL- dependent pathway for separase-independent cohesin removal function during meiosis, as suggested by our preliminary data. Finally, we will conduct a large-scale genetic screen to identify mutations that disrupt SCC mediated by REC-8 cohesin but not COH-3/4 cohesin. This screen can identify cohesin regulators that differentiate between kleisins as well as sequences within kleisins that contribute to their unique properties. Because mutations in cohesin subunits and cohesin regulators are found in numerous types of cancer and are linked to cohesinopathies like Cornelia de Lange and Roberts-SC phocomelia syndromes, the relevance of the proposed research extends beyond reproductive health.